(a) Field of the Invention
The present invention relates to a centrifugally cast double-layer tube which is capable of preventing or mitigating the carbon deposition caused when hydrocarbons or their derivatives are thermally cracked in the presence or absence of steam as seen in ethylene producing apparatus, delayed coking apparatus and the like or when a fluid containing carbon monoxide is treated at elevated temperatures as seen in synthetic gas producing apparatus, which tube also possesses sufficient strength and ductility.
(b) Description of the Prior Art
The tubes used in the above mentioned processes for treating a fluid containing hydrocarbons, their derivatives or carbon monoxide at elevated temperatures, have usually been made of austenitic steels, ferritic steels, austenite.ferrite duplex-phase steels, low-alloy steels and the like. However, these tubes have been defective in that carbon deposition is liable to occur on the surface in contact with the high temperature fluid during the operation, frequently producing various operational evils such as arise in .DELTA.p, lowering in heating efficiency and the like, thereby making it necessary to perform so-called decoking very frequently. It could be said that this impedes the steady running of the apparatus and further acts not only to exert various disadvantages upon the material constituting the apparatus but also aggravates the economy of the process.
In order to prevent the carbon deposition in this apparatus, various investigations have usually been made. For instance, it is reported in "Ind Eng. Chem. Proc.--Design and Development. 8[1](1969) 25 by B. L. Crynes, L. F. Albright" that the carbon deposition in the ethylene producing apparatus can be somewhat suppressed by adding a very small amount of H.sub.2 S to the feed and some processes presently employ this. However the fact is that since the inside of the cracking tube used in the ethylene producing apparatus or the like, is under a very oxidized atmosphere, the effect of the H.sub.2 S was not so great.
Actually, the inner surface of the tube used in the above mentioned apparatus, to which the present invention pertains, is covered with scales. According to the report of Lobo and others (Preprint for the 5th International Congress on Catalysis, Amsterdam (1972)), it is concluded that carbon deposition is caused by the transition metal elements contained in the construction material of the tube member, such as Fe, Ni and the like, and the said deposition is continued because by the atoms of these elements ceaselessly appear, as if floating, on the upper surface of the carbon deposit layer.
Since it is actually proved by the inventors' investigation that when analyzing the coke deposited on the inner surface of the tube member of the apparatus, the transition metal elements such as Fe, Ni and the like can be detected, it is conjectured that carbon deposition is attributable to the supply of transition metal elements such as Fe, Ni and the like caused by reduction of the oxide containing Fe, Ni and the like as its construction elements on the inner surface of the tube, or brought about by the diffusion of transition metal elements through the surface oxide film from the interior of the tube wall.